Figure 1: This FuzzMeasure
waterfall plot shows decay time as well as amplitude versus frequency. Click the image to
see it full size.

By
Dan FitzGerald

I have noticed that quite a number of
people are beginning to use Software and Measurement Microphones to analyse their
listening spaces. The immediate results can be confusing and disappointing. To get the
best from a Room Analysis Tool, some skill and understanding is essential. This is
intended as a simple primer on how to do it, and how to view the results.

WHAT TO EXPECT

No miracles! It is unlikely that you will get beautifully flat
frequency response curves. I find the software is best at showing changes rather than
describing a static situation. It excels at comparative jobs, such as finding the best
speaker and listener positions by trial and measurement. Frequency response is the most
common and easily understood graph. There are other powerful ones also. Particular
frequencies or bands of frequencies which ring on longer than their neighbours will cause
particular notes or tones to stick out, blurring music or speech. This behaviour can be
seen very graphically on the waterfall plot in Figure 1 at left which has three axes. Top

"I
recommend an omni studio recording microphone, or a Sound Level Meter (SLM) with a line
level audio output socket."

"A
sound absorbent panel, temporarily held in place can nuke these problems, also showing
exactly where to put treatment."

"I
tie a thread with a small weight to the microphone. Positioning the dangling weight over
the marked floor spot guarantees repeatable location and height."

The third axis describes time
passing starting from zero at the back. Consider the spectrum at zero as the instant when
the noise is suddenly turned off. The "slices" coming towards the viewer are
spectra taken at later instants. An ideal room would have very even decay, longer at low
frequencies, gradually shortening towards the highs. This 3-dimensional picture is worth a
thousand words to the experienced eye.

Reverb Time (aka RT60,
EDT, T20, T30) can be displayed in octave or third-octave bands as simple bar graphs.
Given time and experimentation, one learns how to interpret these different ways of
visualising the room sound. Changes, however, can always be immediately seen and
appreciated.

THE MICROPHONE

I believe the type of Microphone does not particularly matter for
our purposes. I have used the built-in mic on Laptops and iMacs. Omni is best but not
vital. Rather than buy a "measurement microphone" I recommend an omni studio
recording microphone, or a Sound Level Meter (SLM) with a line level audio output socket.
This socket has a pre-amplified output from the SLM's in-built microphone. I would go for
a modern SLM with a large screen rather than the Radio Shack model. Sound level meters
have many secondary uses and benefits. Also see the article Measuring Microphones.

MEASUREMENT AND EVALUATION TOOLS

ETF 5 is a PC program which most
directly addresses our needs. FuzzMeasure Pro is a similar product
for the Mac. There are many other analysis tools, from free to extremely expensive. I use
several simultaneously to achieve a holistic evaluation of the room.

Our body can generate tones of varied pitch and duration. Uniquely,
this human tone generator can move about the room, tuning in and stimulating hot spots.
Humming, barking, or grunting at varied pitches can provoke the booms, honks, and rings,
and identify exactly where they are. Labmeter linked above will show the frequency. You
might want to be alone when doing this!

A sound absorbent panel, such as a RealTraps MiniTrap or MicroTrap, temporarily held
in place can nuke these problems, also showing exactly where to put treatment.

Lastly, a caveat - some software graphs allows clicking on, say a
peak, to identify it's frequency. These seemingly exact frequencies are not always correct
due to internal math resolution. I wouldn't use them to set a Room EQ for instance. Top

HOW TO DO IT

I am assuming a rectangular room, with speakers at the narrow front
wall. Identify the zones at 3/8 (38%) of room length from the front wall and ditto from
the back wall. These zones mathematically have the best balance of room modes and should
sound best. Another rule of thumb suggests there is little bass at the room centre. These
are useful, often correct, guidelines. However, measurement always trumps theory. Using
masking tape, label the floor at all significant listening spots such as the engineer's
seat, the producer's seat, and the rear couch. Use descriptive names and numbers for your
chosen spots. My software uses one speaker at a time during measurement sweeps, so I use
names like L38FC (Left Speaker Front Centre), L38BL (Left Speaker Back Left), and so on.
Establish your own system and stick to it. Eight spots seems appropriate in a small room.
Mount the microphone or SLM on a stand or tripod. Seated ear height is good. If you use a
mixing desk and like to prowl around, then use standing ear height. Mix and match heights
if you like, but remember to use fully descriptive labels.

I tie a thread with a small weight to the microphone. Positioning
the dangling weight over a spot marked on the floor guarantees repeatable location and
height. I point the microphone directly at the tweeter. 45 degrees or straight up is more
usual Stateside. Whichever you chose, do keep it consistent.

A human body close to the mic causes strong midrange anomalies. So
stay at least a meter away from the microphone. When measuring, the software generates
very loud noises. Wear earplugs or closed headphones. Start at low volume, try a couple of
measurements, increasing the volume until you feel the room is well driven. Watch out for
overload lights on active speakers, particularly at high frequencies. If you have an SLM I
recommend 85-90dB SPL with slow response and C weighting. Label each measurement and move
on. Top

Figure 2: The horizontal
axis is frequency, and vertical is sound level, or amplitude. Click the image to see it
full size.

"ETF
and FuzzMeasure will not tell you simply what is wrong, how to treat your room, where to
put what. They can, however, bestow great certainty when making comparative choices."

HOW
TO VIEW IT

Don't panic! The curves almost always look
awful. Frequency response graphs shows scary peaks and dips. Waterfall plots will often
show one very low extremely long decay, plus a confusing array of peaks and dips up
through the spectrum. Let's take a real world example, shown in Figure 2 at left. This
room is from hell. It has mostly concrete surfaces, it is asymmetrical, and worst of all
it has alcoves. It is a tuned indoor swimming pool.

The green Before curve shows the room with elementary treatment; the
red After curve is with much more considered treatment, of considerable quantity and
quality. We did the lot; four corners, alcove corners, ceiling cloud, RFZ. Sadly, the
After curve has a very similar shape to the Before. Disappointing. Let's look closer, with
focus on the one big issue; the musically crucial 100 Hz zone. Here we find a 6 dB
improvement. Now, consider if you were to EQ a full mix with such a broad 6 dB boost. This
is a big and welcome change. A poll of 7 professional sound engineers was done in this
room. All aspects averaged, the room scored 9 out of 10. Before it was a 6. Perhaps the
curves don't do justice to the great sound and the great change. The frequency plot does
deliver some clear and solid advice though: review the speaker positions. Be careful of
bass decisions in the 100 Hz zone. Try listening at spots in the room where the graph is
flattest. Use top quality headphones to judge kick drum and bass relationships. Top

The waterfall plots told another story. On a cursory glance, the
After again looked very similar to Before in shape, just generally shorter. Octave reverb
time measurements varied a lot with position - some were zero. I am somewhat doubtful as
to the accuracy of reverb time measurements in such a small dead room. I will point out,
however, that once again, zooming in to an area of particular interest clearly illustrates
a spectacular change. The Before waterfall showed a 1.3 second long boom at 37 Hz. After,
it was reduced to 0.7 Seconds. Sonically, this changed a kick drum from a chest massage to
a nice "subby" thump. Note there is almost no sign of this huge anomaly in the
averaged frequency response graph.

CONCLUSIONS

ETF and FuzzMeasure will not tell you simply what is wrong, how to
treat your room, or where to put treatment. They can, however, bestow great certainty when
making comparative choices. Acoustics can be a fascinating and useful study. I find
knowledge of how sound behaves extremely helpful in recording. There is a wealth of
knowledge all over the RealTraps site and elsewhere. There is simple clear advice as to
where to put treatment and why. All the advice agrees on the basics; broadband or bass
traps in the corners, a ceiling cloud, plus side-wall and ceiling reflection points. This
is not voodoo and it doesn't change from room to room. Treat the room first, then use
software and other tools to make informed choices - best speaker position, best seating
positions, best speaker EQ settings, and so forth. Room treatment will yield spectacular
results, no doubt. The use of measurements to decide on positional and other tweaks is the
icing on the cake, not the dough. Top

Sound Engineer "DanDan" FitzGerald began by mixing
at music gigs. This was followed by some international fame for his "hi-fi"
studio and location recordings. In the early 1990s his recording of No Frontiers
by Mary Black took over from Ricky Lee Jones as the test CD in magazines and at
trade shows. DD currently records and mixes Other Voices, a live music TV series.
This year Other Voices was broadcast in HD with 5.1 by RaveHD globally. Recorded
in a tiny church in Dingle at the western edge of Europe, the series recently featured
Ryan Adams and Amy Winehouse. The entire series can be viewed online at Other Voices. Dan is now developing
an acoustics and noise control consultancy SoundSound Acoustics.